The goal of this project is the isolation and characterization of transverse tubules from rabbit ventricular muscle. Transverse tubules are invaginations of the external membrane which make contact with the intracellular CA++ storage system, the sarcoplasmic reticulum (SR). The coupling of plasma membrane excitation and the release of intracellular CA++ for muscle contraction is believed to occur at the junction of the transverse tubule and SR. Transverse tubules, either as joined to SR in dyads or as isolated vesicles, will be isolated from cardiac microsomes by sucrose density gradients in combination with other techniques. The fate of all external membrane vesicles will be tracked by assays for marker proteins such as the Beta-adrenergic receptor and the NaK ATPase. A major identification protocol of the transverse tubule will be through its ability to form an intact dyad. Mechanism of Na+ and CA++ flux such as by ATP energized pumps, Na+ for CA++ exchange and ion channels will be investigated. These studies will lead to an increased understanding of the mechanism of excitation-contraction coupling and the mode of action of cardiac glycosides and other drugs in normal cardiac tissue. The ultimate goal is to determine the changes that occur in ion flux in the transverse tubule during the course of normal development and in diseased states such as cardiac failure.